(1) Field of the Invention
The present invention relates to an optical laser scanning device, and particularly relates to an optical laser scanning device employing a multi-beam method and to an image forming apparatus having the optical laser scanning device as the main component.
(2) Description of Related Art
An image forming apparatus employing the multi-beam method is provided with a semiconductor laser array that is capable of emitting a plurality of laser beams at a time in an optical system. By striking a plurality of scanning lines at a time, the speed of scan processing is improved. Also, by scanning the laser beams on one scanning line, image quality is improved.
There may be three different purposes in emitting the laser beams using the semiconductor laser array. The first purpose is to strike a surface of a photosensitive component according to image data. The second purpose is to detect a Start-of-Scan (referred to as the "SOS" hereinafter). The SOS means a position from which the scanning is started, and this position may be also referred to as the "scanning start position" hereinafter. The third purpose is to perform an Auto Power Control (referred to as the "APC" hereinafter).
The SOS detection is performed such that the scanning start positions according to the image data are aligned for all main scanning lines on the photosensitive component. By means of the SOS detection processing, a timing at which an image based on the image data is written on the surface of the photosensitive component is determined for all main scanning lines.
The APC is performed to avoid the following problem. Due to environmental conditions, such as increased temperature, the light intensity emitted from a laser diode (referred to as the "LD" hereinafter) element varies even under the same current. To avoid this problem, a photo diode detects the intensity of light emitted from the LD element under a predetermined current, and the current to be applied to the LD element is adjusted in accordance with a difference between the detected light intensity and a target value.
Out of the three purposes, the purpose of detecting the SOS further includes two objectives. One is to align the scanning start positions for every scanning process, as is the case of an optical system provided in an image forming apparatus employing a single-beam method. The other is to align the scanning start positions for all laser beams emitted at one time for every scanning process. When the semiconductor laser array employing the multi-beam method is used, the latter objective needs to be particularly performed for a case where phase shifts in the scanning start positions are caused between the laser beams due to the placement of the semiconductor laser array.
The technique for aligning the scanning start positions for all laser beams is disclosed in the U.S. Pat. No. 4,853,710. In this document, the SOS detection is performed for one of the laser beams, and phase shifts in the scanning start positions between the laser beams are corrected by delaying the scanning start timings of the corresponding laser beams. Hereinafter, the laser beam used for detecting the SOS and used as the reference for adjusting the scanning start positions is referred to as the "reference beam."
FIG. 14 is a timing chart showing the timings at which the laser beams are emitted corresponding to the stated three purposes. As shown in FIG. 14, a laser beam is emitted for every scanning line, and a laser beam 1is used as the reference beam. The start timings of emitting the laser beams which strike the surface of the photosensitive component differ from each other (as indicated by SOI1, SOI2, and SOI3in FIG. 14). By means of these different timings, the phase shifts between the laser beams are corrected. The emissions 1411 and 1412 for the APC are performed between an end of the emission onto the photosensitive component (indicated as "EOI3" in FIG. 14) and an emission 1401 for a next SOS detection. It should be noted here that the emission 1401 of the reference beam for the SOS detection is also used for the APC.
In this processing, however, a period of time during which the laser beam used as the reference beam has been emitted is longer than other periods of time during which the other two laser beams have been respectively emitted. This means that the life of a light source in the semiconductor laser array corresponding to the LD element that is used for emitting the reference beam is shortened, as compared with the light sources corresponding to the LD elements that are used for emitting the other laser beams. For this reason, even when the other light sources can be still used, the semiconductor laser array needs replacing if the life of the light source of the reference beam ends. In addition, the maintenance accompanying this replacement is often required.